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CheatSheet.pdf Fixed typo ^_^ Mar 13, 2020

JavaScript CheatSheet

Quick reference to the tremendously accessible high-level language of the web ^_^

The listing sheet, as PDF, can be found here, or as a single column portrait, while below is an unruly html rendition.

This reference sheet is built from a CheatSheets with Org-mode system.

JavaScript is what everyone calls the language, but that name is trademarked (by Oracle, which inherited the trademark from Sun). Therefore, the official name of JavaScript is ECMAScript. The “ECMA” in “ECMAScript” comes from the organisation that hosts the primary standard, the European Computer Manufacturers Association.

As the programming language of browsers, it is remarkably error-tolerant. It simply “fails silently” by giving error values such as undefined when things are not there or 0 / 0 ≈ NaN for nonsensical numeric expressions.

By accident, there are two (mostly) interchangeable values null and undefined that denote the absence of a meaningful value. Many operations that don’t produce meaningful values yield undefined simply because they have to yield some value. Here is a neat story about null.


JavaScript considers types only when actually running the program, and even there often tries to implicitly convert values to the type it expects.

  • typeof gives a string value naming the type of its argument.
  • The functions Number, String, Boolean try to convert values into those types.
console.log(typeof 4.5, typeof '4.5', typeof true)
// ⇒ number string boolean

console.log(8 * null // Multiplication needs numbers so null ↦ 0
             , 'five' * 2 // 'five' is not a number, so 'five' ↦ NaN
             , '5' - 1    // Subtraction needs numbers so '5' ↦ 5
             , '5' + 1)   // The first is a string, 
                          // so “+” denotes catenation, so 1 ↦ '1'

console.log(Number('2.3')   // ⇒ 2.3
           ,Number('five')  // ⇒ NaN
           ,Boolean('five') // ⇒ true
           ,Boolean('')     // ⇒ false
           ,String(NaN)     // ⇒ 'NaN'
           ,String(null))  // ⇒ 'null'

let x₀ = v₀, …, xₙ = vₙ; introduces 𝓃-new names xᵢ each having value vᵢ.

  • The vᵢ are optional, defaulting to undefined.
  • The program crashes if any xᵢ is already declared.
  • Later we use xᵢ = wᵢ; to update the name xᵢ to refer to a new value wᵢ.
    • Augmented updates: x ⊕= y ≡ x = x ⊕ y
    • Increment: x-- ≡ x += 1
    • Decrement: y-- ≡ x -= 1
let x, y = 1, z;
console.log(x, y, z); // ⇒ undefined 1 undefined
  • In the same way, for the same purpose, we may use var but it has undesirable properties; e.g., its declarations are in the global scope and no error is raised using var x = ⋯ if x is already declared.
  • In the same way, we may use const to introduce names that are constant: Any attempt to change their values crashes the program.
  • A binding name may include dollar signs ($) or underscores (_) but no other punctuation or special characters.

Scope and Statements

Each binding has a scope, which is the part of the program in which the binding is visible. For bindings defined outside of any function or block, the scope is the whole program—you can refer to such bindings wherever you want. These are called global.

global bindings are defined outside of any block and can be referenced anywhere.

local bindings are defined within a block and can only be referenced in it.

let, const declare local bindings; var always makes global ones!

Besides the assignment statement, we also have the following statements:

  • Conditionals: if (condition) A else B
  • Blocks: If Sᵢ are statements, then {S₀; …; Sₙ;} is a statement.
  • The for/of syntax applies to arrays, strings, and other iterable structures —we will define our own later.
    // Print all the elements in the given list.
    for (let x of ['a', 1, 2.3]) {
      console.log(`x ≈ ${x}`);

JavaScript is whitespace insensitive.


In addition to the standard arithmetic operations, we have Math.max(x₀, …, xₙ) that takes any number of numbers and gives the largest; likewise Math.min(⋯). Other common functions include Math.sqrt, Math.ceil, Math.round, Math.abs,~ and ~Math.random() which returns a random number between 0 and 1. Also, use % for remainder after division.

// Scientific notation: 𝓍e𝓎 ≈ 𝓍 × 10ʸ
console.log(1, 2.998e8, 100 + 4 * 11)

// Special numbers so that division “never crashes”.
console.log(1/0, -1/0, Infinity - 10) // ⇒ Infinity -Infinity Infinity
console.log(Infinity - Infinity, 0/0) // ⇒ NaN NaN

// Random number in range min...Max
Math.floor(Math.random() * (max - min) + min)

NaN stands for “not a number”, it is what you get when a numeric expression has no meaningful value.

  • Any NaN in an arithmetic expressions swallows the whole expression into a NaN.
  • Number.isNaN(x) is true iff x is NaN.

Everything is equal to itself, except NaN. Why? NaN denotes the result of nonsensical computations, and so is not equal to the result of any other nonsensical computation.

console.log(NaN == NaN) // ⇒ false


The empty string =”=, list [], and 0, NaN are falsey —all else is truthy.

  • Note: (p < q < r) ≈ (p < q) < r, it is not conjunctive!
console.log(true, false, 3 > 2, 1 < 2, 1 != 2, 4 <= 2 < 3)

// Upper case letters come first, then lower case ones.
console.log('abc' < 'def', 'Z' < 'a')

// Equality with coercions, and without.
console.log(1.23 == '1.23', 1.23 === '1.23')
  • Precise Equality === is equality with no type coercions.
  • Applying the “not” ! operator will convert a value to Boolean type before negating it.
  • Precedence: Relationals like == and > are first, then “and” &&, then “or” ||.
  • The ternary operator: condition ? if_true : if_false

Empty Values

console.log(null == undefined) // ⇒ true

Only the empty values are coerced into being equal, no other value is equal to an empty value. As such, x != null means that x is not an empty value, and is in fact a real meaningful value.

Since && and || are lazy, x || y means return x if x != false and otherwise return y; i.e., give me x if it’s non-empty, else y.

Likewise, x && y means give me y, if x is nonempty, else give me the particular empty value x.

console.log( 4  == 3   && 4   // 3 is truthy 
           ,''  == ''  && 4   // '' is falsey
           ,'H' == 'H' && 4   // 'H' is truthy
           , 0  == 0   && 4   // 0 is falsey
           , 4  == 0   || 4   // 0 is falsey

Any pair of matching single-quotes, backticks, or double-quotes will produce a string literal. However, backticks come with extra support: They can span multiple lines and produce formatted strings, where an expression can be evaluated if it is enclosed in ${⋯}.

console.log(`half of 100 is ${100 / 2}`) // ⇒ half of 100 is 50

String Methods

  • s.repeat(𝓃) ≈ Get a new string by gluing 𝓃-copies of the string 𝓈.
  • Trim removes spaces, newlines, tabs, and other whitespace from the start and end of a string.
    console.log("  okay \n ".trim()); // ⇒ okay
  • s.toUpperCase() and s.toLowerCase() to change case.
  • 𝓈.padStart(𝓁, 𝓅) ≈ Ensure 𝓈 is of length ≥ 𝓁 by padding it with 𝓅 at the start.
    console.log(String(6).padStart(3, "0")); // ⇒ 006
  • 𝓈.replace(/./g, c => p(c) ? f(c) : '') ≈ Keep only the characters that satisfy predicate p, then transform them via f.
    let s = 'abcde'.replace(/./g, c => 'ace'.includes(c) ? c.toUpperCase() : '')
    console.log(s); // ⇒ ACE

Method Names Shared with Arrays

The following methods also apply to arrays.

  • s.length ⇒ Length of string
  • s[𝒾] ⇒ Get the 𝒾-th character from the start
    • Unless 0 ≤ 𝒾 < s.length, we have s[𝒾] = undefined.
  • s.concat(t) ⇒ Glue together two strings into one longer string; i.e., s + t.
    console.log(('cat' + 'enation').toUpperCase()) // ⇒ CATENATION
  • s.includes(t) ⇒ Does s contain t as a substring?
  • s.indexOf(t) ⇒ Where does substring t start in s, or -1 if it’s not in s.
    • To search from the end instead of the start, use lastIndexOf.
  • s.slice(𝓂,𝓃) ⇒ Get the substring between indices 𝓂 (inclusive) and 𝓃 (exclusive).
    • 𝓃 is optional, defaulting to s.length.
    • If 𝓃 is negative, it means start from the end: s.slice(-𝓃) ≈ s.slice(s.length - 𝓃).
    • s.slice() ⇒ Gives a copy of s.

Treating Strings as Arrays

  • There is no character type, instead characters are just strings of length 1.
  • You can “split” a string on every occurrence of another string to get a list of words, and which you can “join” to get a new sentence. 𝓈.split(𝒹).join(𝒹) ≈ 𝓈.
  • To treat a string as an array of characters, so we can apply array only methods such as f = reverse, we can use split and join:
  • Keeping certain characters is best done with regular expressions.


Array indexing, arr[𝒾], yields the 𝒾-th element from the start; i.e., the number of items to skip; whence arr[0] is the first element.

(Stack) The push method adds values to the end of an array, and the pop method does the opposite, removing the last value in the array and returning it. (Queue) The corresponding methods for adding and removing things at the start of an array are called unshift and shift, respectively.

Arrays have the following methods, which behave similar to the string ones from earlier.


One difference is that unlike string’s indexOf, which searches for substrings, array’s indexOf searches for a specific value, a single element.

console.log([1, 2, 3, 2, 1].indexOf(2));     // ⇒ 1
console.log([1, 2, 3, 2, 1].lastIndexOf(2)); // ⇒ 3

The concat method can be used to glue arrays together to create a new array, similar to what the + operator does for strings.

  • If you pass concat an argument that is not an array, that value will be added to the new array as if it were a one-element array. This is a push.

Array(𝓃).fill(𝓍) ≈ Get a new array of 𝓃-copies of element 𝓍.

Array methods

  • xs.forEach(a) to loop over the elements in an array and perform action a.
  • xs.filter(p) returns a new array containing only the elements that pass the predicate p.
  • transforms an array by putting each element through the function f.
  • xs.reduce(f, e) combines all the elements in an array into a single value.
    • We can omit the starting value e if the array xs is non-empty, in which case e is taken to be the first element xs[0].
  • xs.some(p) tests whether any element matches a given predicate function p.
    • xs.every(p) tests if every element of xs satisfies p.
  • xs.findIndex(p) finds the position of the first element that matches the predicate p.

With the exception of forEach, the above functions do not modify the array they are given.

// Print the elements of the given array
[`a`, `b`, `c`].forEach(l => console.log(l));

// ∃/∀
console.log([1, 2, 3].some(e => e == 2))  // true
console.log([1, 2, 3].every(e => e == 2)) // false

// Sum the elements of an array
console.log([1, 2, 3, 4].reduce((soFar, current) => soFar + current)); // ⇒ 10

// flatten an array of arrays
let flatten = (xss) => xss.reduce((sofar, xs) => sofar.concat(xs), [])

let arrays = [[1, 2, 3], [4, 5], [6]]; 
console.log(flatten(arrays)) // ⇒ [ 1, 2, 3, 4, 5, 6 ]

Higher-order functions start to shine when you need to compose operations.


Function values can do all the things that other values can do; i.e., they can be used in arbitrary expressions; e.g., a binding that holds a function is still just a regular binding and can, if not constant, be assigned a new value.

  • A function definition is a regular binding where the value of the binding is a function.

    Functions declared using the top-level function keyword may be used before their declarations.

// Shorter way to define functions
function square2(x) {
  return x * x;
  • A return keyword without an expression after it will cause the function to return undefined.
  • Functions that don’t have a return statement at all, similarly return undefined.
  • Declaring function f (⋯) {⋯} will not raise a warning if the name f is already in use —similar to var.
  • One may also define functions using “arrow” notation: (x₀, …, xₙ) => ⋯.
    • When there is only one parameter name, you can omit the parentheses around the parameter list.
    • If the body is a single expression, rather than a (multi-line) block in braces, that expression will be returned from the function.

    So, these two definitions of square do the same thing:

    const square1 = (x) => { return x * x; };
    const square2 =  x  => x * x;

As will be seen, arrow functions are not exactly the same as declared functions.

JavaScript is extremely fault-tolerant: If we give a function more arguments than it needs, the extra arguments are just ignored. If we give it too few arguments, the missing arguments are assigned undefined.

// Extra arguments are ignored
console.log(square(4, true, "hedgehog")); // ⇒ 16

// No longer a function!
square = 'g'

Default Values

(Default Values) If you write an = operator after a parameter, followed by an expression, the value of that expression will replace the argument when it is not given.

let square = (x = 1) => x * x;
console.log(square(3)); // ⇒ 9
console.log(square());  // ⇒ 1

Rest Parameters

(Rest Parameters) It can be useful for a function to accept any number of arguments. For example, Math.max computes the maximum of all the arguments it is given. To write such a function, you put three dots before the function’s last parameter, which is called “the rest parameter” and it is treated as an array containing all further arguments.

You can use a similar three-dot notation to call a function with an array of arguments.

let numbers = [5, 1, 7];
// ⇒ 7

This “spreads” out the array into the function call, passing its elements as separate arguments. It is possible to include an array like that along with other arguments, as in max(9, ...numbers, 2).

Higher-order Functions

Higher-order functions allow us to abstract over actions, not just values. They come in several forms.

For example, we can have functions that create new functions.

let greaterThan   = n => (m => m > n);
let greaterThan10 = greaterThan(10);
console.log(greaterThan10(11)); // ⇒ true

And we can have functions that change other functions. (Decorators)

function noisy(f) {
  return (...args) => {
    let result = f(...args);
    console.log(`Called: ${}(${args}) ≈ ${result}`);
    return result;
noisy(Math.min)(3, 2, 1); // Called: min(3,2,1) ≈ 1

We can even write functions that provide new types of control flow.

let n = 8;
unless(n % 2 == 1, () => {
    console.log(n, "is even");
// ⇒ 8 is even

Destructuring and the “spread” Operator

If you know the value you are binding is an array/object, you can use []/{} brackets to “look inside” of the value, binding its contents.

One of the reasons the doit function below is awkward to read is that we have a binding pointing at our array, but we’d much prefer to have bindings for the elements of the array, whence the second definition of doit.

let xs = [9, 11, 22, 666, 999];

// The following are the same.
function doit(xs){ return xs[0] + xs[1] + xs[2]; }
function doit([x, y, z]) {return x + y + z; }
// Only first three items accessed in “doit”; extra args are ignored as usual.

// Destructuring to get first three elements and remaining
let x = xs[0], y = xs[1], z = xs[2], ws = xs.slice(3);
console.log(x, y, z, ws) // ⇒ 9 11 22 [ 666, 999 ]
// Nice! Same thing.
let [a, b, c, ...ds] = xs 
console.log(a, b, c, ds) // ⇒ 9 11 22 [ 666, 999 ]

// Destructuring to get first and remaining elements
let [head, ...tail] = xs
console.log(head, tail) // ⇒ 9 [ 11, 22, 666, 999 ]

// Destructuring on an object to get two properties and the remaining subobject 
let {name, age, ...more} = {name: "Musa", age: 72, x: 1, y: 2}
console.log(name, age, more) // ⇒ Musa 72 { x: 1, y: 2 }

// Destructuring: Simultaneous assignment!
var p = 1, q = 2    // ⇒ 1, 2
var [p, q] = [q, p] // swap them
console.log(p, q)   // ⇒ 2, 1

// Unpacking: f(...[x₀, …, xₙ]) ≈ f(x₀, …, xₙ)
console.log(Math.min(...xs)) // ⇒ 9

// Unpacking: Merging arrays/objects
let ys = [1, ...xs, 2, 3] // ⇒ 1, 9, 11, 22, 666, 999, 2, 3
let zs = {w: 0, ...more, z: 3} // ⇒ { w: 0, x: 1, y: 2, z: 3 }

// Updating a property, a key-value pair
zs = {...zs, w: -1} // ⇒ { w: -1, x: 1, y: 2, z: 3 }

Note that if you try to destructure null or undefined, you get an error, much as you would if you directly try to access a property of those values.

   let {x₀, …, xₙ, ...𝓌} = 𝓋                                       
≡ let x₀ = 𝓋.x₀, …, xₙ = 𝓋.xₙ; 𝓌 = 𝓋; delete 𝓌.x₀, …, delete 𝓌.x

As usual, in arrow functions, we may destructure according to the shape of the elements of the array; e.g., if they are lists of at least length 2 we use (soFar, [x, y]) => ⋯. This may be useful in higher order functions such as map, filter, reduce.


Objects and arrays (which are a specific kind of object) provide ways to group several values into a single value. Conceptually, this allows us to put a bunch of related things in a bag and run around with the bag, instead of wrapping our arms around all of the individual things and trying to hold on to them separately. These “things” are called properties.

Arrays are just a kind of object specialised for storing sequences of things.

Object Creation

Values of the type object are arbitrary collections of properties. One way to create an object is by using braces as an expression that lists properties as “name:value” pairs.

  1. Almost all JavaScript values have properties. The exceptions are null and undefined. If you try to access a property on one of these nonvalues, you get an error. Properties are accessed using value.prop or value["prop"].
  2. Whereas value.x fetches the property of value named x, value[e] tries to evaluate the expression e and uses the result, converted to a string, as the property name.
  3. The dot notation only works with properties whose names look like valid (variable) binding names. So if you want to access a property named 2 or John Doe, you must use square brackets: value[2] or value["John Doe"].
  4. Unless value contains a property x, we have value.x ≈ undefined.
    • Hence, out of bounds indexing results in undefined.
  5. Notice that the this keyword allows us to refer to other parts of this object literal. Above, info used the person object’s information, whereas speak did not. The “this” keyword is covered in more detail below.
  6. Variables names in an object literal, like languages, denote a shorthand for a property with the same and value, but otherwise is no longer related to that binding.

    This is useful if we want multiple objects to have the same binding; e.g., with let x = ⋯, a = {name: 'a', x}, b = {name: 'b', x}, both objects have a x property: a.x and b.x.

  7. We cannot dynamically attach new properties to the atomic types String, Number, Boolean; e.g., let x = 2; x.vest = 'purple'; console.log(x.vest); prints undefined. We can write it, but they “don’t stick”.
  8. Below, we could have begun with the empty object then added properties dynamically: let person = {}; = `musa`; person.age = 29; ….
let languages = ['js', 'python', 'lisp']
let person = { name: 'musa'
             , age: 27
             , 'favourite number': 1
             , languages // Shorthand for “languages: ['js', 'python', 'lisp']”
             , age: 29  // Later bindings override earlier ones.
             // Two ways to attach methods; the second is a shorthand.
             , speak: () => `Salamun Alaykum! Hello!`
             , info () { return `${} is ${this.age} years old!`; }

console.log(person.age) // ⇒ 29

// Trying to access non-existent properties
// Reading a property that doesn’t exist will give you the value undefined.
console.log(person.height) // ⇒ undefined
// Is the property “name” in object “person”?
console.log('name' in person); // ⇒ true

// Updating a (computed) property
let prop = 'favourite' + ' ' + 'number'
person['favourite number'] = 1792       
console.log(person[prop]) // ⇒ 1792

// Dynamically adding a new property
person.vest = 'purple'
console.log(person.vest) // ⇒ purple

// Discard a property
delete person['favourite number']

// Get the list of property names that an object *currently* has.
console.log(Object.keys(person)) // ⇒ [ 'name', 'age', 'languages', 'vest' ]

// Variables can contribute to object definitions, but are otherwise unrelated.
languages = ['C#', 'Ruby', 'Prolog']
console.log(person.languages) // ⇒ [ 'js', 'python', 'lisp' ]

// Calling an object's methods
console.log(person.speak()) // ⇒ Salamun Alaykum! Hello!
console.log(  // ⇒ musa is 29 years old!

Getters and Setters – get, set

You can define getters and setters to secretly call methods every time an object’s property is accessed. E.g., below num lets you read and write value as any number, but internally the getter method is called which only shows you the value’s remainder after division by the modulus property.

let num = { modulus: 10
          , get value() { return this._secret % this.modulus; }
          , set value(val) { this._secret = val; } }
console.log(num.value) // ⇒ 9
num.modulus = 12;
console.log(num.value) // ⇒ 3
  • Exercise: Make an object num such that num.value varies, returning a random number less than 100, each time it’s accessed.

Using get, set is a way to furnish prototypes with well-behaved properties that are readable or writable, or both.

Dictionries or maps

An object can also be used as a “key:value” dictionary: When we ‘look-up’ a key, we find a particular value. E.g., with ages = {mark: 12, james: 23, larry: 42} we use ages.mark to find Mark’s age.

Similarly, objects can be used to simulate keyword arguments in function calls.

The this Keyword

Usually a method needs to do something with the object it was called on. When a function is called as a method — looked up as a property and immediately called, as in object.method() —– the binding called this in its body automatically points at the object that it was called on.

function speak(line) {
  console.log(`The ${this.type} rabbit says '${line}'`);
let whiteRabbit  = {type: "white", speak};
let hungryRabbit = {type: "hungry", speak};

whiteRabbit.speak("Hola!"); // ⇒ The white rabbit says 'Hola!'
hungryRabbit.speak("Hey!")  // ⇒ The hungry rabbit says 'Hey!'


You can think of this as an extra parameter that is passed in a different way. If you want to pass it explicitly, you can use a function’s call method, which takes the this value as its first argument and treats further arguments as normal parameters., "Burp!");
// ⇒ The hungry rabbit says 'Burp!'

With call, an object can use a method belonging to another object. E.g., below we use whiteRabbit’s speaking method with its this keywords referring to exoticRabbit.

let exoticRabbit = {type: 'exotic'}, `Jambo!`) 
// ⇒ The exotic rabbit says 'Jambo!'

The this of Parent Scopes

Since each function has its own this binding, whose value depends on the way it is called, you cannot refer to the this of the wrapping scope in a regular function defined with the function keyword.

Arrow functions are different —they do not bind their own this but can see the this binding of the scope around them. Thus, you can do something like the following code, which references this from inside a local function:

function normalise() {
  console.log( => n / this.length));
}{coords: [0, 2, 3], length: 5}); // ⇒ [0, 0.4, 0.6]

If we had written the argument to map using the function keyword, the code wouldn’t work.

Object-Oriented Programming

In English, prototype means a preliminary model of something from which other forms are developed or copied. As such, a prototypical object is an object denoting the original or typical form of something.

In addition to their properties, JavaScript objects also have prototype —i.e., another object that is used as a source of additional properties. When an object gets a request for a property that it does not have, its prototype will be searched for the property, then the prototype’s prototype, and so on.

  • Object.getPrototypeOf(x) returns the prototype of an object x.

For example, arrays are derived from Array.prototype which is derived from Object.prototype —which is the great ancestral prototype, the entity behind almost all object. Object.prototype provides a few methods that show up in all objects, such as toString, which converts an object to a string representation.

  • We can use the Object.getOwnPropertyNames(x) to get all the property names linked to object x.

It is occasionally useful to know whether an object was derived from a specific class. For this, JavaScript provides a binary operator called instanceof. Almost every object is an instance of Object.

  • 𝓍 instanceof 𝓎 ≈ Object.getPrototypeOf(𝓍) == 𝓎.prototype
 // “Object” includes “toString”, and some other technical utilities.

// Some true facts
console.log( {}       instanceof Object
           , []       instanceof Array
           , Math.max instanceof Function
           , Math.max instanceof Object) // Since Function derives from Object

// “Object” has no parent prototype.
console.log(Object.getPrototypeOf(Object.prototype)); // ⇒ null

Adding New Methods or Overriding Methods

(Extension Methods / Open Classes) To attach a new property to a ‘kind’ of object, we simply need to attach it to the prototype —since all those ‘kinds’ of objects use the prototype’s properties. Let’s attach a new method that can be used with any array.

Array.prototype.max = function () { 
   console.log('ola'); return Math.max(...this)

console.log([3,1,5].max()); // ⇒ Prints “ola”, returns 5

// ⇒ Includes length, slice, …, and our new “max” from above

When you call the String function (which converts a value to a string) on an object, it will call the toString method on that object to try to create a meaningful string from it.

Array.prototype.toString = function() { return this.join(' and '); };
console.log(String([1, 2, 3])) // ⇒ 1 and 2 and 3

(Overriding) When you add a property to an object, whether it is present in the prototype or not, the property is added to the object itself. If there was already a property with the same name in the prototype, this property will no longer affect the object, as it is now hidden behind the object’s own property.

xs.colour = 'green'
console.log(xs.colour) // ⇒ green

// ⇒ purple 

Inheritance and Null Parents

You can use Object.create to create an object with a specific prototype. The default prototype is Object.prototype. For the most part, Object.create(someObject) ≈ { ...someObject }; i.e., we copy the properties of someObject into an empty object, thereby treating someObject as a prototype from which we will build more sophisticated objects.

Unlike other object-oriented languages where Object sits as the ancestor of all objects, in JavaScript it is possible to create objects with no prototype parent!

// Empty object that *does* derive from “Object”
let basic = {}
console.log( basic instanceof Object // ⇒ true
           , "toString" in basic)    // ⇒ true

// Empty object that does not derive from “Object”
let maximal = Object.create(null);
console.log( maximal instanceof Object // ⇒ false
           , "toString" in maximal)    // ⇒ false

⟨1⟩ Prototype Example

Prototypes let us define properties that are the same for all instances, but properties that differ per instance are stored directly in the objects themselves. E.g., the prototypical person acts as a container for the properties that are shared by all people. An individual person object, like kathy below, contains properties that apply only to itself, such as its name, and derives shared properties from its prototype.

// An example object prototype
let prototypicalPerson    = {};
prototypicalPerson._world = 0;
prototypicalPerson.speak  = function () {
  console.log(`I am ${}, a ${this.job}, in a world of `
               + `${prototypicalPerson._world} people.`) }
prototypicalPerson.job = `farmer`;

// Example use: Manually ensure the necessary properties are setup
// and then manually increment the number of people in the world.
let person = Object.create(prototypicalPerson); = `jasim`;
person.speak() // ⇒ I am jasim, a farmer, in a world of 1 people.

// Another person requires just as much setup
let kathy = { ...prototypicalPerson }; // Same as “Object.create(⋯)” = `kathy`;
kathy.speak() // ⇒ I am kathy, a farmer, in a world of 2 people.

⟨2⟩ Manual Constructor Functions

Classes are prototypes along with constructor functions!

A class defines the shape of a kind of object; i.e., what properties it has; e.g., a Person can speak, as all people can, but should have its own name property to speak of. This idea is realised as a prototype along with a constructor function that ensures an instance object not only derives from the proper prototype but also ensures it, itself, has the properties that instances of the class are supposed to have.

let prototypicalPerson    = {};
prototypicalPerson._world = 0;
prototypicalPerson.speak  = function () {
  console.log(`I am ${}, a ${this.job}, in a world of `
               + `${prototypicalPerson._world} people.`) }

function makePerson(name, job = `farmer`) {
  let person  = Object.create(prototypicalPerson); = name;
  person.job  = job;
  return person;

// Example use
let jasim = makePerson(`jasim`);
jasim.speak() // I am jasim, a farmer, in a world of 1 people.
makePerson(`kathy`).speak() // I am kathy, a farmer, in a world of 2 people.

⟨3⟩ Constructor Functions with new

We can fuse these under one name by making the prototype a part of the constructor.

  • By convention, the names of constructors are capitalised so that they can easily be distinguished from other functions.
function Person(name, job = `farmer`) { = name;
 this.job  = job;

Person.prototype._world = 0;
Person.prototype.speak = function () {
  console.log(`I am ${}, a ${this.job}, in a world of `
               + `${Person.prototype._world} people.`) }

// Example use
let jasim = Object.create(Person.prototype), `jasim`)
jasim.speak() // ⇒ I am jasim, a farmer, in a world of 1 people.

// Example using shorthand
let kasim = new Person (`kathy`)
kasim.speak()  // ⇒ I am kathy, a farmer, in a world of 2 people.

If you put the keyword new in front of a function call, the function is treated as a constructor. This means that an object with the right prototype is automatically created, bound to this in the function, and returned at the end of the function.

  new f(args)                                                                   
≈ (_ => let THIS = Object.create(f.prototype);, args); return THIS;) ()

All functions automatically get a property named prototype, which by default holds a plain, empty object that derives from Object.prototype. You can overwrite it with a new object if you want. Or you can add properties to the existing object, as the example does.

Notice that the Person object derives from Function.prototype, but also has a property named prototype which is used for instances created through it.

console.log( Object.getPrototypeOf(Person) == Function.prototype
           , Person instanceof Function
           , jasim  instanceof Person
           , Object.getPrototypeOf(jasim) == Person.prototype)

Hence, we can update our motto:

Classes are constructor functions with a prototype property!

⟨4⟩ class Notation

Rather than declaring a constructor, then attaching properties to its prototype, we may perform both steps together using class notation shorthand.

class Person {
  static #world = 0
  constructor(name, job = `farmer`) { = name;
    this.job  = job;
  speak() {
    console.log(`I am ${}, a ${this.job}, in a world of `
               + `${Person.#world} people.`) 

// Example use

let jasim = new Person(`jasim`)
// ⇒ I am jasim, a farmer, in a world of 1 people.

new Person(`kathy`).speak()
// ⇒ I am kathy, a farmer, in a world of 2 people.

Notice that there is a special function named constructor which is bound to the class name, Person, outside the class. The remainder of the class declarations are bound to the constructor’s prototype. Thus, the earlier class declaration is equivalent to the constructor definition from the previous section. It just looks nicer.

  • Actually, this is even better: The static #world = 0 declaration makes the property world private, completely inaccessible from the outside the class. The static keyword attaches the name not to particular instances (this) but rather to the constructor/class name (Person).
  • Indeed, in the previous examples we could have accidentally messed-up our world count. Now, we get an error if we write Person.#world outside of the class.

The Iterator Interface

The object given to a for/of loop is expected to be iterable. This means it has a method named Symbol.iterator. When called, that method should return an object that provides a second interface, the iterator. This is the actual thing that iterates. It has a next method that returns the next result. That result should be an object with a value property that provides the next value, if there is one, and a done property, which should be true when there are no more results and false otherwise.

Let’s make an iterable to traverse expression trees.

class Expr { // [0] Our type of expression trees
  static Constant(x) { 
     let e = new Expr();
     e.tag = 'constant', e.constant = x; 
     return e; 

  static Plus(l, r) { 
     let e = new Expr();
     e.tag = 'plus', e.left = l, e.right = r;
     return e; 

// [1] The class tracks the progress of iterating over an expression tree
class ExprIterator {
  constructor(expr) { this.expr = expr; this.unvisited = [{expr, depth: 0}]; }
  next () {
   if(this.unvisited.length == 0) return {done: true};
   let {expr , depth} = this.unvisited.pop();
   if (expr.tag == 'constant') return {value: {num: expr.constant, depth}}
   if (expr.tag == 'plus') {
     // pre-order traversal
     this.unvisited.push({expr: expr.right, depth: depth + 1})
     this.unvisited.push({expr: expr.left, depth: depth + 1})

// [2] We can add the iterator after-the-fact rather than within the Expr class.
Expr.prototype[Symbol.iterator] = function () { return new ExprIterator(this) }

// [3] Here's some helpers and an example.
let num = (i) => Expr.Constant(i)
let sum = (l, r) => Expr.Plus(l, r)
// test ≈ 1 +  (2 + (3 + 4))
let test = sum( num(1), sum( num(2), sum(num(3), num(4))))
// console.log(test) // ⇒ Nice looking tree ^_^

// [4] We now loop over an expression with for/of
for (let {num, depth} of test)
  console.log(`${num} is ${depth} deep in the expression`)

Recall that inside a class declaration, methods that have static written before their name are stored on the constructor. It appears that static properties are shared by all instances, because the constructor object has these as properties rather than particular instance objects.

JavaScript and the Browser

Browsers run JavaScript programs, which may be dangerous and so browsers limit what a program may do —e.g., it cannot look at your files.

An HTML document is a nested sequence of tagged items, which may be interpreted as a living data-structure —with the screen reflecting any modifications.

  • The most important HTML tag is <script>. This tag allows us to include a piece of JavaScript in a document.

The data-structure is called the Document Object Model, or DOM, and it is accessed with the variable document.

  • The DOM interface wasn’t designed for just JavaScript; e.g., it can be used with XML.

Call the following snippet test.html, then open it in your favourite browser.

<title> Ola! </title>

<h3 id="myHeader"></h3>

alert(`Welcome to my webapge!`);

let count = 0;
function modifyThePage(){
  document.title = `New zany title ~ ${Math.random()}`;
  myHeader.innerText = `New zany heading ~ ${count}`;
  count += Math.floor(Math.random() * 10);

<button onclick="modifyThePage();">Change the title and header </button>

Such a script will run as soon as its <script> tag is encountered while the browser reads the HTML. This page will pop up a dialog when opened to show a message.

Some attributes can also contain a JavaScript program. The <button> tag shows up as a button and has an onclick attribute whose value (function) will be run whenever the button is clicked.

Notice that by providing ID’s to tags, we may refer to them in our JavaScript code.

Large Scripts

Including large programs directly in HTML documents is often impractical. The <script> tag can be given a src attribute to fetch a script file (a text file containing a JavaScript program) from a URL.

<h1>Testing alert</h1>
<script src="code/hello.js"></script>

The code/hello.js file included here contains the simple program alert("hello!").


Some Fun Stuff ^_^

// A “quine” is a program that prints itself, such as this one:
f = _ => console.log(`f = ${f};f()`); f()
// Prints:
// f = _ => console.log(`f = ${f};f()`);f()

// Range of numbers. Including start, excluding end.
let range = (start, end) => [...Array(end - start).keys()].map(x => x + start)
console.log(range(3, 8)) // ⇒ [ 3, 4, 5, 6, 7 ]

// Flatten an array
let xss     = [[1, 2, 3], [4, 5, 6]]
let flatten = xss => [].concat(...xss)
console.log(flatten(xss)) // ⇒ [ 1, 2, 3, 4, 5, 6 ]

// Randomise the elements of an array
let shuffle = (arr) => arr.slice().sort(() => Math.random() - 0.5)
let xs = [1, 2, 3, 4, 5, 6] 
console.log(shuffle(xs)) // ⇒ [ 5, 1, 4, 6, 2, 3 ]
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